ixgbe: Fix - Do not allow Rx FC on 82598 at 1G due to errata

The 82598 has an erratum that receipt of pause frames at 1G
could lead to a Tx Hang.  To avoid this this patch disables
Rx FC while at 1G speed for all 82598 parts.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>